≤10 % of modern biomass
||Schönheit P, Buckel W, Martin WF. On the Origin of Heterotrophy. Trends Microbiol. 2016 Jan24(1):12-25. doi: 10.1016/j.tim.2015.10.003. p.13 3rd paragraphPubMed ID26578093
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||P.13 3rd paragraph:"Cells: Much Better than Stardust and Mostly Protein-A logical consequence of autotrophic origins is that anaerobic autotrophs were not only the ancestors of the
first heterotrophs, they were also the
first viable substrate for heterotrophic
growth (Box 1). Critics might interject that there were huge amounts of reduced carbon compounds delivered to Earth from space [ref 35], and that such material also could have served
as viable substrate for heterotrophs. Organics from space unquestionably did accumulate on the early Earth, but could they have been a viable substrate? The answer is no. Organics delivered from space, despite comprising as much as perhaps 10% of the Earth's modern biomass by weight [primary source], can be excluded as a likely first substrate for heterotrophs because they are chemically too heterogeneous."